The goals of this research project are to explore the mechanisms and pathways encoding frequency modulated (FM) sounds, acoustic elements that are very important in communication, particularly in human speech, where they comprise the formant transitions in different phonemes. We will study how FM sounds are encoded in the auditory systems of the mustached bat, a species with a sonar signal resembling format transitions and a vocal communications repertoire rich in FM components. We have previously discovered a discrete region of the external nucleus of the inferior colliculus (ICXv) in which neurons are particularly sensitive to FM stimuli. ICXv is a previously unrecognized component of the "central acoustic tract" (CAT), a poorly understood pathway directly connecting the brainstem with the auditory thalamus, bypassing the midbrain. The proposed studies will examine 1) whether FM selectivity arises prior to the ICXv in the brainstem nucleus of the CAT (NCAT); 2) whether selectivity is elaborated in the suprageniculate nucleus of the auditory thalamus, where both ICXv and NCAT send their outputs; 3) how cells in both locations respond to FM stimuli at various sweep rates, durations, amplitudes and directions: 4) how excitatory and inhibitory inputs interact temporality to determine FM rate and direction selectivity; 5) how the nuclei in the pathway are anatomically interconnected; and 6) how the inhibitory neurotransmitters glycine and GABA are distributed in the pathway. These experiments will clarify how the processing of FM stimuli is elaborated in the central acoustic tract. We also hope eventually to elucidate the mechanisms that might underlie formant transition processing for speech in the human brain, and perhaps contribute to the understanding of phonological disorders in children that impair language acquisition.